The production of light weight, shaped objects, such as auto body parts, which have high impact strength, that can be laid up or molded into complex shapes, and that will retain dimensional stability over a wide variety of use conditions has been an enduring goal of plastics researchers and manufacturers for several decades. Recently, great advances towards this goal have been made using reinforced plastics or composites, in which two or more layers of resinous sheeting, sometimes using fibrous reinforcement, are laminated together to form a laminated composite wherein the physical properties of the resulting laminate exceed what would be expected considering the properties of the individual layers.
Some of the most encouraging results have been achieved with laminates employing reinforced resin layers in conjunction with an unreinforced layer of a different resin. In the area of high performance composites, i.e., as for aircraft panels, U.S. Pat. No. 4,539,253, to Hirschbuehler, et al, discloses an interleafed fiber resin matrix composition wherein a reinforced thermoset epoxy resin sheet is layered with a fiber-reinforced interleaf resin including a thermoset epoxy and a rubbery vinyl addition polymer.
U.S. Pat. No. 2,719,100 to Banigan discloses a process for heat-sealing thermoplastic laminates, that is, bonding together (by heat) the component layers of thermoplastic films, e.g., poly(ethylene terephthalate), PET, by interposing a substantially amorphous thermoplastic film between adjacent layers of tensilized, i.e., stretched, film to be heat-sealed. The heat-sealing process results in a strong, light-weight bonded film useful for packaging. However, because the process requires tensilization to make the laminated film, it is difficult to produce such films having a thickness substantially greater than 0.007" in a continuous type of stretching apparatus. Thus, thickness is a limiting factor.
Kennedy, in U.S. Pat. No. 3,357,874, describes a process for laminating polyester films and other "addends" onto the surface of a shaped polyester article by treating the surface with an acid wash (e.g., 85% sulfuric acid), to leave the surface in an amorphous condition. When the amorphous surface of the film is brought into contact with an addend material, a laminate is formed having a strong interface adhesion between the component layers. However, the use of an acid treatment is costly and makes the substrates difficult to handle.
U.S. Pat. Nos. 3,798,116 and 3,969,176, to Bassett, et al, describe a method for preparing bonded polyester films having a bead type heat seal between the plies of the composite film.
U.S. Pat. No. 4,041,206, to Tsunashima, et al, teaches that PET or poly(butylene terephthalate), PBT, films can be laminated directly to a crystalline poly(butylene terephthalate) or poly(hexylene terephthalate) copolyester blended with 10-40 weight percent PET or PBT, said copolyester containing 50-80 mole percent terephthalic acid units. The resulting films are transparent, tough, slippery and have excellent heat-adhesive properties, making them useful in general packaging, photographic films and electrical insulation. However, no mention is made of the suitability of this system for reinforced, dimensionally stable objects.
U.S. Pat. No. 4,314,002 to Oizumi, et al, discloses circuit board laminates comprising alternating fiber-reinforced curable thermoset resin layers and unreinforced cured resin layers in which the same or a different resin may be used in both types of layers. The reinforced layers, e.g., linter paper or kraft paper impregnated with a thermoset resin, are separated by cured resin layers, forming an integral laminate in which voids between layers due to contraction during curing are eliminated.
U.S. Pat. No. 4,373,002 to Peterson-Hoj discloses a heat-sealable laminated material comprising a layer of stretched crystalline polyester and a layer of cyclohexane-modified, heat-sealable amorphous polyester material, which layers are joined by lamination or coextrusion and then subjected to a joint stretching operation. The resulting laminate is heat-sealable and also exhibits high tensile strength.
The foregoing patents demonstrate that considerable work has been done in the area of resin laminates, both reinforced and unreinforced, which obtain advantageous properties by promoting, in various ways, close bonding between the respective layers, to give an integral composite. There is still a strong need, however, for reinforced composites utilizing thermoplastic resins to produce articles having high impact strength.
It has now been surprisingly discovered that unique, interleafed fiber-reinforced thermoplastic composites can be formed using layers of a fiber-reinforced thermoplastic resin separated by layers of a ductile thermoplastic interleaf copolymer resin. The composites of the present invention are distinguished from foregoing composites in that the interleaf block copolymer resin is compatible with the fiber-reinforced thermoplastic resin so as to undergo a co-crystallization or co-vitrification, resulting in chemical interlayer bonding which has not been seen in prior art laminates. Such co-crystallization is achieved without any of the surface treatment techniques (acid treatment, tensilization, precuring, etc.) seen in prior processes.
The interleaf copolymer resin forms a ductile, tough, rubbery layer, and, in the final composite of the invention, will form a diffuse interface with the fiber-reinforced, or "binder", resin. The final thermoplastic composites have high impact strength and high resistance to delamination.
While not intending to be bound by any theory of operation, it is believed that the following factors may be important in providing the advantageous results obtained with the present invention:
(i) Adhesion between the reinforced substrate and the interlayer occurs because the binder resin in the substrate and the hard blocks (hereinafter (a)) in the block copolymer of the interlayer mix with a negative Gibbs free energy. This may be a consequence of a co-crystallizability or of some other thermodynamic driving force, such as negative mixing enthalpy or positive entropic energy.
(ii) The soft blocks (hereinafter (b)) in the block copolymer of the interlayer always have low Tg, in fact always less than 25.degree. C. and typically less than 50.degree. C. This does not guarantee that they will be incompatible with the binder resin in the composite substrate, thus demixing during cool down. They must be chosen on rigorous thermodynamic considerations to insure demixing.